This application is related to Japanese Patent Application No. 2001-122754 filed in Apr. 20, 2001, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, and more particularly, it relates to a ladder type surface acoustic wave filter having plural surface acoustic wave resonators arranged in a series-arm and a parallel-arm.
2. Description of the Related Art
A ladder type surface acoustic wave (hereinafter referred to as SAW) filter using plural SAW resonators has been known as a band pass filter (see, for example, Journal of Institute of Electronics, Information and Communication Engineers A, Vol. J 76-A, No. 2, pp. 245-252 (1993)).
FIG. 1 shows a constitutional diagram of a conventional ladder type SAW filter.
A ladder type SAW filter has such a constitution that series-arm SAW resonators S1 and S2 are arranged between an input terminal Ti and an output terminal To of a piezoelectric substrate, and parallel-arm SAW resonators P1 and P2 are arranged between the input and output terminals and a ground terminal G. SAW resonators S1, S2, P1, P2, are generally called a one-port SAW resonators.
FIG. 2 shows a constitutional diagram of a one-port SAW resonator. A one-port SAW resonator has such a constitution that an interdigital transducer (hereinafter referred to as IDT) for electrically exciting SAW and reflectors arranged on a propagation path of the SAW for trapping the excited SAW in the IDT are formed on the piezoelectric substrate. In the case where resonant properties are obtained by utilizing internal reflection of the SAW by the IDT itself, the reflectors are not arranged on the substrate.
The IDT is formed with a large number of electrode fingers arranged in a comb form with a constant period (pi).
The reflectors are formed with a large number of grating electrode fingers arranged with a constant period (pr) and is also referred to as a grating reflector.
In the IDT, the SAW is excited by two adjacent electrode fingers each extending downward and upward as one unit.
A ladder type SAW filter is designed in such a manner that a resonance frequency frs of the series-arm SAW resonators S1 and S2 substantially agrees to an antiresonance frequency fap of the parallel-arm SAW resonators P1 and P2.
FIG. 3(a) is a graph showing a pass characteristic diagram of a ladder type SAW filter, and FIG. 3(b) is a graph showing single impedance characteristic diagrams at this time of the series-arm SAW resonators S1 an S2 and the parallel-arm SAW resonators P1 and P2. The graph g1 in FIG. 3(b) is a graph of the series-arm SAW resonators S1 and S2, and the graph g2 is a graph of the parallel-arm SAW resonators P1 and P2. In the graph g1 of the series-arm SAW resonators, the frequency where the impedance becomes minimum is the resonance frequency frs, and the frequency where the impedance becomes maximum is the antiresonance frequency fas.
FIG. 4 is an explanatory diagram of frequency characteristics demanded for a band pass filter, such as a ladder type SAW filter.
The characteristic values herein include demanded pass band widths (BW1 and BW2), attenuation levels (ATT1 and ATT2) defined by the specification, and attenuation band widths (BWatt1 and BWatt2).
A ratio (BW1/BW2) of the band widths BW2 and BW1 at a certain attenuation level is referred to as a shape factor. The closer the shape factor to 1, it is better and referred to as a high shape factor. The shape factor of the ladder type SAW filter is substantially defined by the frequency difference between the resonance frequency fr and the antiresonance frequency fa.
That is, the steepness of the inclination from the attenuation region on the low frequency side to the pass region depends on the frequency difference (xcex94fp in FIG. 3(b)) between the resonance frequency frp and the antiresonance frequency fap of the parallel-arm resonators P1 and P2, and the smaller the xcex94fp is, the steeper the inclination is. The steepness of the declination from the pass region to the attenuation region on the high frequency side depends on the frequency difference (xcex94fs in FIG. 3(b)) between the resonance frequency frs and the antiresonance frequency fas of the series-arm resonators S1 and S2, and the smaller the xcex94fs is, the steeper the declination is.
However the xcex94fp and xcex94fs are almost determined by the electromechanical coupling factor of a piezoelectric substrate used and do not change if the number of electrode pairs or an overlap width of electrodes of the IDT is changed. In this respect, Japanese Unexamined Patent Publication No. HEI 8(1996)-23256 and Japanese Patent Application No. HEI 10(1998)-514149 disclose SAW resonators whose xcex94fp and xcex94fs are decreased by withdrawing more electrodes from end portions of the IDT than from a central part.
Japanese Unexamined Patent Publication No. Hei 11(1999)-163664 discloses an SAW filter having xcex94fp and xcex94fs that are made small by periodically withdrawing the electrode fingers of the IDT. The xcex94fp and the xcex94fs can be adjusted with a simple constitution by using the withdrawing methods, so as to realize a ladder type SAW filter having a desired pass band width and steepness of the edge parts of the pass region.
However, in the case where the electrode fingers of the IDT are periodically withdrawn, the xcex94fp (xcex94fs) is decreased to realize a filter of a high shape factor, but such a problem arises that spurious occurs outside the pass band of the filter. The spurious outside the pass band occurs at plural positions corresponding to the period, at which the electrode fingers of the IDT are withdrawn, and there are many cases where the attenuation characteristics demanded outside the pass band of the filter cannot be satisfied.
In the case where the electrode fingers of the IDT are withdrawn in a larger number in the outside of the IDT, on the other hand, spurious occurring outside the pass band can be suppressed, and the Dfp (Dfs) can also be decreased, whereby a filter of a high shape factor having good attenuation characteristics outside the pass band can be realized. However, according to the configuration, the reduction ratio of the Dfp (Dfs) is poor with respect to the number of the withdrawal electrode fingers among the total number of the electrode fingers of the IDT (withdrawing ratio R), and thus the reduction ratio of the Dfp (Dfs) becomes about half of that obtained by withdrawing the electrode fingers periodically with the same value of R.
Therefore, in order to produce the same xcex94f (shape factor), this procedure requires that the value of R be necessarily about twice that in the procedure where the electrode fingers are periodically withdrawn. Because the electrostatic capacitance is decreased by withdrawing the electrode fingers, the aperture length of the IDT is increased or the number of pairs of the electrode fingers is increased to compensate the reduction in capacitance. In other words, when the value of R is increased twice, the area of the IDT is increased twice or more to cause such a problem that the filter chip size cannot be decreased.
The invention provides a surface acoustic wave resonator comprising a piezoelectric substrate having formed thereon at least one interdigital transducer, the at least one interdigital transducer comprising a prescribed number of interdigitated electrode fingers, at least two electrode fingers among the interdigitated electrode fingers being withdrawn out, and positions of the withdrawal electrode fingers being aperiodic.
According to the invention, the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator can be made close to each other, whereby a compact surface acoustic wave filter having a high shape factor of the surface acoustic wave filter with suppressed spurious outside the pass band can be provided.